1. Field of the Invention
This invention relates to an apparatus for transmitting an output with predetermined frequency response and waveform shape over a communications media, more particularly to a waveform shaping circuit and method employed in such an apparatus for transmitting Manchester-encoded data to an unshielded twisted-pair media, the waveform shaping circuit and method having a relatively low power consumption and being relatively inexpensive to implement.
2. Description of the Related Art
A conventional apparatus for transmitting Manchester-encoded data to an unshielded twisted-pair media has been disclosed in U.S. Pat. No. 5,267,269 issued to Shih et al. The conventional apparatus includes: a sequencer for receiving data in a non-return-to-zero format and for providing a mode select output and a step select output in response thereto; a memory for storing data representing a plurality of predetermined waveforms; a multiplexer having a first input coupled to the step select output, a second input coupled to the mode select output, an input bus coupled to the memory and an output bus for providing data representing a select waveform from the memory in response to the step and mode select outputs; a latch having an input coupled to the output bus and an output for de-skewing the data representing the select waveform; a differential digital-to-analog converter having an input coupled to an output of the latch and providing a differential analog current output proportional to the select waveform; and driver means including low pass filter means having a differential input coupled to the current output for impressing an analog voltage onto a transmission line.
Although the apparatus disclosed in the aforementioned U.S. Patent provides higher integration level and some cost saving due to elimination of a passive filter, it occupies significant silicon area and consumes large power in view of its complexity. Furthermore, additional circuits are required to generate the high oversampling clocks as only the fundamental bit rate clock is available in the application.